MPLS S. Bryant
Internet-Draft G. Swallow
Intended status: Standards Track S. Sivabalan
Expires: September 3, 2015 Cisco Systems
March 2, 2015
RFC6374 Synonymous Flow Labels
draft-bryant-mpls-synonymous-flow-labels-00
Abstract
This document describes a method of providing flow identification
information when making RFC6374 performance measurements. This
allows RFC6374 measurements to be made on multi-point to point LSPs
and allows the measurement of flows within an MPLS construct using
RFC6374.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 3, 2015.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Synonymous Flow Labels . . . . . . . . . . . . . . . . . . . 3
4. User Service Traffic in the Data Plane . . . . . . . . . . . 4
4.1. Applications Label Present . . . . . . . . . . . . . . . 4
4.2. Single Label Stack . . . . . . . . . . . . . . . . . . . 5
5. RFC6374 Packet Loss Measurement . . . . . . . . . . . . . . . 7
5.1. SFL TLV . . . . . . . . . . . . . . . . . . . . . . . . . 8
6. Manageability Considerations . . . . . . . . . . . . . . . . 9
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
[I-D.bryant-mpls-flow-ident] describes the requirement for
introducing flow identities when using RFC6374 [RFC6374] packet loss
measurements. In summary RFC6374 uses the RFC6374 packet as the
packet accounting demarcation point. Unfortunately this gives rise
to a number of problems that may lead to significant packet
accounting errors:
1. Where a flow is subjected to Equal Cost Multi-Path (ECMP)
treatment packets may arrive out of order with respect to the
RFC6374 packet.
2. Where a flow is subjected to ECMP treatment packets may arrive at
different hardware interfaces, thus requiring reception of an
RFC6374 packet on one interface to trigger a packet accounting
action on another interface which may not be co-located with it.
This is a difficult technical problem to address with the
required degree of accuracy.
3. Even where there is no ECMP (for example on RSVP-TE, MPLS-TP LSPs
and PWs) local processing may be distributed over a number of
cores, leading to synchronization problems.
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4. Some forwarder implementations have a long pipeline between
processing a packet and incrementing the associated counter.
An approach to mitigating these synchronization issue is described in
[I-D.tempia-opsawg-p3m] in which packets are batched by the sender
and each batch is marked in some way such that adjacent batches can
be easily recognized by the receiver.
An additional problem arises where the LSP is a multi-point to point
LSP, since MPLS does not include a source address in the packet.
Network management operations require the measurement of packet loss
between a source and destination. It is thus necessary to introduce
some source specific information into the packet to identify packet
batches from a specific source.
This document describes a method of accomplishing this by using a
technique called synonymous flow labels Section 3 in which labels
which mimic the behaviour of other labels provide the packet batch
identifiers and enable the per batch packet accounting.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. Synonymous Flow Labels
A synonymous flow label (SFL) is defined to be a label that causes
exactly the same forwarding behaviour at the egress Label Switching
Router (LSR) as another label, except that it also causes an
additional agreed action to take place on the packet. In this
application the agreed action is the recording of the receipt of the
packet by incrementing a packet counter. This is a natural action in
many MPLS implementations, and where supported this permits the
implementation of high quality packet loss measurement without any
change to the packet forwarding system.
Consider an MPLS application such as a pseudowire (PW), and consider
that it is desired to use the approach specified in this document to
make a packet loss measurement. By some method outside the scope of
this text, two labels, synonymous with the PW labels are obtained
from the egress terminating provider edge (T-PE). By alternating
between these SLs and using them in place of the PW label, the PW
packets may be batched for counting without any impact on the PW
forwarding behaviour (note that strictly only one SL is needed in
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this application, but that optimization is a matter for the
implementor).
Now consider an MPLS application that is multi-point to point such as
a VPN. Here it is necessary to identify a packet batch from a
specific source. This is achieved by making the SLs source specific,
so that batches from one source are marked differently from batches
form another source. Note that the sources all operate independently
and asynchronously from each other, independently co-ordinating with
the destination.
Finally we need to consider the case where there is no MPLS
application label such as occurs when sending IP over an LSP. In
this case introducing an SL that was synonymous with the LSP label
would introduce network wide forwarding state. This would not be
acceptable for scaling reasons. We therefore have no choice but to
introduce an additional label. Where penultimate hop popping (PHP)
is in use the semantics of this additional label can be similar to
the LSP label. Where PHP is not in use he semantics are similar to
an MPLS explicit NULL. In both cases with the additional semantics
of the SL.
Note that to achieve the goals set out in Section 1 SLs need to be
allocated from the platform label table.
4. User Service Traffic in the Data Plane
As noted in Section 3 it is necessary to consider two cases:
1. Applications label present
2. Single label stack
4.1. Applications Label Present
Figure 1 shows the case in which both an LSP label and an application
label is present in the MPLS label stack. Uninstrumented traffic
runs over the "normal" stack, and instrumented flows run over the SFL
stack with the SFL used to indicate the packet batch.
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+-----------------+ +-----------------+
| | | |
| LSP | | LSP | <May be PHPed
| Label | | Label |
+-----------------+ +-----------------+
| | | |
| Application | | Synonymous Flow |
| Label | | Label |
+-----------------+ +-----------------+
| | | |
| Payload | | Payload |
| | | |
+-----------------+ +-----------------+
"Normal" Label Stack Label Stack with SFL
Figure 1: Use of Synonymous Labels In A Two Label MPLS Label Stack
At the egress LSR the LSP label is popped (if present). Then the SFL
is processed in exactly the same way as the corresponding application
label would have been processed. Where the SFL is being used to
support RFC6374 packet loss measurements, as an additional operation,
the total number of packets received with this particular SFL is
recorded.
4.2. Single Label Stack
Figure 2 shows the case in which only an LSP label is present in the
MPLS label stack. Uninstrumented traffic runs over the "normal"
stack and instrumented flows run over the SFL stack with the SFL used
to indicate the packet batch. However in this case it is necessary
for the ingress LSR to first push the SFL and then to push the LSP
label.
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+-----------------+
| |
| LSP | <= May be PHPed
| Label |
+-----------------+ +-----------------+
| | | |
| Application | | Synonymous Flow | <= Explicit NULL
| Label | | Label |
+-----------------+ +-----------------+ <= Bottom of stack
| | | |
| Payload | | Payload |
| | | |
+-----------------+ +-----------------+
"Normal" Label Stack Label Stack with SFL
(Mode 1) (Mode 3)
Figure 2: Use of Synonymous Labels In A Single Label MPLS Label Stack
At the receiving LSR it is necessary to consider two cases:
1. Where the LSP label is still present
2. Where the LSP label is penultimate hop popped
If the LSP label is present, it processed exactly as it would
normally processed and then it is popped. This reveals the SFL which
in the case of RFC6374 measurements is simply counted and then
discarded. In this respect the SFL is synonymous with an explicit
NULL. As the SFL is the bottom of stack, the IP packet that follows
is processed as normal.
If the LSP label is not present due to PHP action in the upstream
LSR, two almost equivalent processing actions can take place. Either
the SFL can be treated as an LSP label that was not PHPed and then
take the additional associated SFL action, which in this case is
packet batch counting, or it can be treated as an explicit NULL with
associated SFL actions. From the perspective of the measurement
system described in this document the behaviour of two approaches are
indistinguishable and thus either may be implemented.
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5. RFC6374 Packet Loss Measurement
The packet format of an RFC6374 Query message using SFLs is shown in
Figure 3.
+-------------------------------+
| |
| LSP |
| Label |
+-------------------------------+
| |
| Synonymous Flow |
| Label |
+-------------------------------+
| |
| |
| RFC6374 Measurement Message |
| |
| +-------------------------+ |
| | | |
| | RFC6374 Fixed | |
| | Header | |
| | | |
| +-------------------------+ |
| | | |
| | Optional SFL TLV | |
| | | |
| +-------------------------+ |
| | | |
| | Optional Return | |
| | Information | |
| | | |
| +-------------------------+ |
| |
+-------------------------------+
Figure 3: RFC6734 Query Packet with SFL
The MPLS label stack is exactly the same as that used for the user
data service packets being instrumented (see Section 4). The RFC6374
measurement message consists of the three components, the RFC6374
fixed header as specified in [RFC6374] carried over the ACH channel
type specified the type of measurement being made (currently: loss,
delay or loss and delay) as specified in RFC6374.
Two optional TLVs MAY also be carried if needed. The first is the
SFL TLV specified in Section 5.1. This is used to provide the
implementation with a reminder of the SFL that was used to carry the
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RFC6374 message. This is needed because a number of MPLS
implementations do not provide the MPLS label stack to the MPLS OAM
handler. This TLV is required if RFC6374 messages are sent over UDP
(draft-bryant-mpls-RFC63740-over-udp). This TLV MUST be included
unless, by some method outside the scope of this document, it is
known that this information is not needed by the RFC6374 Responder.
The second set of information that may be needed is the return
information that allows the responder send the RFC6374 response to
the Querier. This is not needed if the response is requested in-band
and the MPLS construct being measured is a point to point LSP, but
otherwise MUST be carried. The return address TLV is defined in
RFC6378 and the optional UDP Return Object is defined in
[I-D.ietf-mpls-rfc6374-udp-return-path].
5.1. SFL TLV
The SFL TLV is shown in Figure 4. This contains the SFL that was
carried in the label stack, the FEC that was used to allocate the SFL
and the index into the batch of SLs that were allocated for the FEC
that corresponds to this SFL.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |MBZ| SFL Batch | SFL Index |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SFL | FEC >
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
> FEC cont |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: SFL TLV
Where:
Type Type is set to Synonymous Flow Label (SFL-TLV).
Length The length of the TLV as specified in [RFC6374].
MBZ MUST be sent as zero and ignored on receive.
SFL Batch The SFL batch that this SFL was allocated as part of
(see draft-bryant-mpls-sfl-control)
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SPL Index The index into the list of SPLs that were assigned
against the FEC that corresponds to the SPL.
SFL The SPL used to deliver this packet. This is an MPLS
label which is a component of a label stack entry as
defined in Section 2.1 of [RFC3032].
FEC The Forwarding Equivalence Class that was used to
request this SPL. This is encoded as per
Section 3.4.1 of
This information is needed to allow for operation with hardware that
discards the MPLS label stack before passing the remainder of the
stack to the OAM handler. By providing both the SFL and the FEC plus
index into the array of allocated SFLs a number of implementation
types are supported.
6. Manageability Considerations
This will be considered in a future version of this document.
7. Privacy Considerations
The inclusion of originating and/or flow information in a packet
provides more identity information and hence potentially degrades the
privacy of the communication. Whilst the inclusion of the additional
granularity does allow greater insight into the flow characteristics
it does not specifically identify which node originated the packet
other than by inspection of the network at the point of ingress, or
inspection of the control protocol packets. This privacy threat may
be mitigated by encrypting the control protocol packets, regularly
changing the synonymous labels and by concurrently using a number of
such labels.
8. Security Considerations
The system described in this memo introduces no additional security
vulnerabilities.
9. IANA Considerations
IANA is request to allocate a new TLV from the 0-127 range on the
MPLS Loss/Delay Measurement TLV Object Registry:
Type Description Reference
---- --------------------------------- ---------
TBD Synonymous Flow Label This
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A value of 4 is recommended.
10. Acknowledgements
TBD
11. References
11.1. Normative References
[I-D.ietf-mpls-rfc6374-udp-return-path]
Bryant, S., Sivabalan, S., and S. Soni, "RFC6374 UDP
Return Path", draft-ietf-mpls-rfc6374-udp-return-path-02
(work in progress), September 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
11.2. Informative References
[I-D.bryant-mpls-flow-ident]
Bryant, S. and C. Pignataro, "MPLS Flow Identification",
draft-bryant-mpls-flow-ident-00 (work in progress),
October 2014.
[I-D.tempia-opsawg-p3m]
Capello, A., Cociglio, M., Castaldelli, L., and A. Bonda,
"A packet based method for passive performance
monitoring", draft-tempia-opsawg-p3m-04 (work in
progress), February 2014.
[RFC6374] Frost, D. and S. Bryant, "Packet Loss and Delay
Measurement for MPLS Networks", RFC 6374, September 2011.
Authors' Addresses
Stewart Bryant
Cisco Systems
Email: stbryant@cisco.com
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George Swallow
Cisco Systems
Email: swallow@cisco.com
Siva Sivabalan
Cisco Systems
Email: msiva@cisco.com
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